The present invention generally relates to a CATV system including a scrambling operation which disables subscribers, other than specified subscribers, from receiving programs. More particularly, the present invention relates to a CATV system in which a main head end as a system center is connected to subhead ends respectively corresponding to a plurality of terminal unit groups through optical fiber cables .which are used as telephone lines and have a narrow transmission band width.
In the CATV system, subscribers having contracts for receiving chargeable programs pay charges every program or every month. Therefore, it is necessary to take measures for disabling persons without contracts and persons trying to wiretap. As one of these measures, there is a method in which trap devices for preventing the passage of chargeable programs are disposed in lead-in wires to subscribers and the devices trap for contracted subscribers are removed so as to enable them to receive pay programs. However, in this method, chargeable programs are wiretapped relatively easily.
On the other hand, there is another method in which a certain signal processing technique known as "scrambling" is performed on the signal-transmitting side so as to make it impossible to receive chargeable programs by a general TV receiver. The demodulation of a scrambled signal is called "descrambling." In realizing the scrambling operation, it is necessary to perform, utilizing principles or natures of the TV receiver, the signal processing for preventing the signal reproduction, and further necessary to sufficiently consider preservation of image quality, secrecy, economical efficiency and reliability of terminal units, etc.
The most common scrambling method is the "gray-sync" type in which synchronization portions are suppressed to disturb the image. This type is generally classified into two systems an analog processing system and a digital processing system.
In the analog processing system, the signal level is continuously changed such that horizontal synchronization portions are compressed and signal portions are elongated. More specifically, the video signal is re-modulated by a sine wave having such a phase relation that the horizontal synchronization portions are compressed. With this processing, not only the horizontal synchronization is disturbed, but also the color signals are affected so that color tones are considerably shifted. In this analog gray-sync processing system, the modulating and demodulating processings are made relatively simple, but the image is also deteriorated as it is processed.
In contrast to this, in the digital processing system, the signal portions are not processed at all and only the synchronization portions are compressed. The digital processing system includes a base band processing method and a high frequency processing method. Since the signal portions are not processed in this system, the image is theoretically not deteriorated. However, in the actual circuitry, influences of switching circuits appear in the demodulated image. An advantage of this system is that because there are a lot of digital circuit included, it is possible to integrate the circuits to a high degree, thereby reducing the cost of the terminal unit.
In a conventional CATV system as shown in FIG. 1, optical fiber cables 3 already being in use as telephone lines are employed as signal transmission lines from a main head end (MHE) 1 as a system center to subhead ends (SHE) 2.sub.1 to 2.sub.3 (which are called 2.sub.n in the following description). This constitution reduces the number of repeaters which must be disposed on the signal transmission lines from the main head end 1 to many terminal unit groups 4.sub.1 to 4.sub.3 (which are called 4.sub.n in the following description) far away from the main head end 1. In this system, coaxial cables 6 are used as signal transmission lines from each subhead end 2.sub.n to many subscribers 5 in each terminal unit group 4.sub.n. However, since the transmission band width of the existing optical fiber cables 3 (telephone lines) is narrow, the TV signal must be transmitted in the form of a baseband signal.
In the case of the scrambling by the digital gray-sync processing system mentioned above, the scrambling operation is performed by suppressing the synchronization portions. Accordingly, when the scrambling operation is performed with respect to the baseband signal before being transmitted from the main head end 1, this baseband signal received in the subhead end 2n cannot be converted into a high frequency signal (RF signal having an IF frequency) since there are no synchronization signals included having sufficient amplitude in the baseband signal. Therefore, it is not appropriate to transmit from the main head end the baseband signal with the compressed synchronization portions.
Therefore, conventionally, in realizing the scrambling operation by the digital gray-sync processing system, an encoder for performing the scrambling operation is disposed in each of the subhead ends 2.sub.n corresponding to the respective terminal unit groups 4.sub.n. After the scrambling operation is performed by the encoder, the TV signal is transmitted to the subscribers 5 through the coaxial cables 6.
FIG. 2 is a block diagram showing the construction of the main head end 1 and subhead end 2.sub.n in the conventional CATV system mentioned above. The main head end 1 has a source 1a such as a video tape recorder and video disk player for generating the baseband TV signal to be transmitted, a computer 1b for producing various kinds of data to be transmitted together with the TV signal, and an electro-optical (E/O) converter 1c for converting the electric signal to an optical signal so as to be transmitted through the optical fiber cable 3.
The subhead end 2.sub.n has an opto-electric (O/E) converter 2a for converting the optical signal transmitted through the optical fiber cable 3 to an electric signal, an amplifier 2b for amplifying the electric signal from the O/E converter 2a, and an E/O converter 2c for converting the electric signal amplified by the amplifier 2b into an optical signal and transmitting the optical signal to the optical fiber cable 3 disposed between the adjacent subhead ends 2.sub.n.
The subhead end 2.sub.n also has a timing generator 2d as a scrambling data generation means for extracting a synchronization signal from the TV signal in the electric signal provided through the conversion in the above O/E converter 2a and generating scramble data on the basis of the extracted synchronization signal; a TV modulator 2e for modulating the baseband TV signal transmitted through the optical fiber cable 3 to produce an IF signal (RF signal) having an intermediate frequency; and a data modem 2f for demodulating the data generated by the computer 1b in the main head end 1 and transmitted together with the baseband TV signal, and modulating the demodulated data so as to be superimposed on the IF signal from the television modulator 2e. Furthermore, the subhead end 2.sub.n has a scrambler 2g as a scramble means for performing the scrambling operation, and a TV up-converter 2h.
According to the above digital signal processing system the scrambler 2g scrambles the IF signal outputted from the television modulator 2e on the basis of the scrambling data generated by the timing generator 2d. The scrambled IF signal is up-converted by the TV up-converter 2h to a signal having a frequency of the second or third channel which is receivable by a TV receiver, and is transmitted to the coaxial cable 6 together with the data from the data modem 2f.
In such a conventional CATV system mentioned above, the entire encoder including the timing generator 2d for generating the scrambling data and the scrambler 2g is installed in each subhead end 2.sub.n in a similar way. Accordingly, the number of encoders is the same as the number of subhead ends 2.sub.n, thereby resulting in a high cost of the entire CATV system.